1. Field of the Invention
The invention is directed to a fiber wrap design, and more particularly to a rotary cable wrap which communicates a cable or cables from a stationary base to a rotating element.
2. Background of the Invention
Rotary cable wraps for the communication of signals via cable from a fixed to a movable, or rotating point are used in a variety of applications, where a rotating receiver, such as a radio receiving antenna is used, mounted to a fixed base unit. Other applications include robotics, such as a manipulator arm rotating about a fixed base, for example.
Other technology exists for transmitting signals from a rotational portion to a fixed portion. A slip ring is often used to transmit signals or power to a number of different destinations. Slip rings are generally either drum style or pancake style. In either case, an element called a rotor interacts with an element called a brush to allow electrical signals to pass between them. Slip rings are generally suitable for communicating electrical signals, but are not intended for communicating (fiber) optical signals. Due to the increased capacity of fiber optics, many more channels/signals can be sent than with traditional electrical wiring, and so the slip ring is not suitable where fiber optics is used.
Whether a slip ring or other electrical connection is used to transmit signals over a rotating joint, there is a risk of data loss or signal degradation at the switch joint. Additionally, these joints are complex, expensive to repair and maintain, and may not be necessary for certain applications.
A light wave traveling in free space has an electric field which is always orthogonal to the propagation direction (z-axis). The oscillations of the electric field are always transverse, with Ez(t)=0. When the light wave is transmitted in an optical waveguide, such as fiber, this relation is not true. However, for weakly-guiding structures, such as fiber, Ez(t)=0 is still a good approximation to only consider the transverse components of the field, Ex(t) and Ey(t). The vector {Ex(t), Ey(t)} is called the Jones Vector. The Jones Vector defines unambiguously the state of polarization (SOP) of the light wave. Another popular description of the SOP of light waves is the Stokes vector. For geometrical representation, the Polarization Ellipse and Poincare Sphere are often used. More details about the definitions and relations of these parameters for representing the SOP of light waves can be found elsewhere in text books on optics (for example, Born and Wolf, Principles of Optics).
For a Local Oscillator (LO) reference signal, attenuation and chromatic dispersion may be respectively accounted for by selecting the correct launching power and using a highly stable phase-locked source. Fiber nonlinearity effects on an LO signal have not been rigorously studied so far, but given low peak optical power in the transmission, nonlinearity will not be a serious problem. In addition to these effects, the Polarization mode dispersion (PMD) can disturb a transmitted light wave.
PMD arises from the anisotropic nature of the fiber/waveguide cross-section. In general, two orthogonal polarization modes are supported in a fiber. The slight asymmetries cause the light in the two polarization modes to travel at slightly different speeds. PMD denotes the effect of the different group propagation velocities of the fast and slow components of the signal. The effects arise from the intrinsic PMD caused by the non-circular core due to fabrication and the cabling processes, and the extrinsic PMD caused by external factors such as the external mechanical and thermal stress. The inherent asymmetries of the fiber are fairly constant over time, while the mechanical stress due to the movement of the fiber can vary, resulting in a dynamic change in the PMD.
Due to the fiber asymmetries, the group delay along a fiber is a function of the polarization of the input signal. If the input light is coupled both into the fast- and slow-axes of the local fiber section (whether Polarization maintaining (PM) or single mode (SM) fiber), the wave will split and propagate at two different velocities. Depending on the distribution of asymmetries along the fiber length, the group velocities of fast- and slow-axes, and the output SOP can change.
For a short fiber section without varying external perturbation or a short optical waveguide based component, a uniform elliptical core along its length can be assumed. Therefore only intrinsic PMD appears. Although the output SOP will change as a function of the input SOP, wavelength and fiber length, there is no power transformation between the fast and slow components. (The power transformation is called mode coupling). If the light is launched with an input SOP aligned to one of the principal axes of such a uniform optical waveguide, the waveguide can be treated similarly to a PM fiber, simply because there is no coupling between components polarized along the fast and slow axes. In such a short fiber/waveguide, the Differential Group Delay (DGD) between the fast- and slow-axes is constant with time, and wavelength. In this case the PMD is deterministic. The short fiber acts like a birefringent crystal with a fixed PMD value. The DGD increases linearly with the fiber length, providing the fiber is kept straight, is not twisted, is kept from varying tension and stress, and its length is short. The relation between the DGD value and the fiber length is described by the PMD coefficient. The intrinsic PMD coefficient for a short piece of telecom SM fiber depends strongly on the fiber type, and can be characterized by its beat length, i.e., the distance needed for a phase difference of 2π between polarization modes. Beat lengths of SM fiber range from a few centimeters in older fibers to meters in today's telecom fiber, the latter corresponding to a PMD coefficient of fs/m. High birefringence fiber (HBF), such as Panda fiber which has a PMD coefficient of 1-2 ps/m can be used as PM fiber, and has a beat length of the order of a millimeter. Meanwhile low birefringence fiber (LBF) also exists on the market, and this has a beat length of longer than 50 m. This type of fiber is manufactured with near perfect circular cores and has been used as PM fiber over short lengths in component manufacture.
When a short fiber is bent uniformly along its length, the perturbation induced can become dominant over intrinsic factors. The PMD in this case increases linearly with the length, and also as a function of the bending. The bending induced PMD coefficient varies depending upon fiber type. Values around 10/R (fs cm/360° turn) are expected, where R is the radius of bending given in cm. For a SM28 fiber with 10 cm bending radius, 0.17 fs/m is a typical value at 1550 μm.
The SOP of the light traveling inside SM fibers is very sensitive to external stresses. With less than one meter of SM fiber, one SOP can be converted to another SOP without significant bending/twisting of the fiber. A relative delay in the x- and y- components as small as 1.5 fs is enough to convert from a linear SOP to a circular SOP. Practically, this is used to make polarization converters (polarization controllers).
For a long length of fiber or a short fiber but with irregular perturbation, the birefringence along its length varies owing to manufacturing variations and externally applied perturbations originating from the bends, twists, stresses and temperature changes in the fiber. These perturbations are usually random along the fiber length. As a result, the polarization will rotate and couple in different proportions between the fast and slow axes. Some of the power launched in the fast polarization mode couples into the slow mode in later lengths of the fiber and vice versa. These random mode couplings tend to equalize the propagation times of the two polarization modes, thereby reducing PMD. For long telecom fiber with random coupling, the PMD coefficient is given in units of ps/km1/2, as the PMD increases as the square root of length. Methods to reduce the PMD coefficient include decreasing the fiber birefringence during manufacture or increasing the mode coupling by using techniques such as twisting the fiber with several twists per meter.
The PMD is often closely associated with the term Differential Group Delay (DGD). DGD is defined by the time delay between the components along the fast and slow Principal States of Polarization (PSPs). In a long fiber, the PSPs are just the SOPs where the light travels at its fastest and slowest. In PM fiber, the PSPs correspond to the linear SOPs along the fast and slow axes. For a long fiber link with random mode coupling, the DGD is instantaneous and varies randomly with wavelength and time.
The average of the DGD distribution is defined by the ITU standard bodies as the PMD value. The average DGD measured over time or wavelength results in the same PMD value, according to random mode coupling theory. Measurement of the time average is, however, generally impractical and therefore the wavelength average is normally used. Therefore, a PMD value is independent of time and wavelength, as the PMD value is the result of an average over a long time or wide wavelength range. The value of the PMD of a fiber is referred to as first-order PMD.
Second-order PMD is defined as the DGD dependency on wavelength. The Second-order PMD includes the Polarization dependent Chromatic Dispersion (PCD), which is the magnitude of the DGD changes with wavelength, and the Depolarization Rate, which describes the rotation of the DGD or PSP. If the first-order PMD is reduced towards zero, second-order PMD is generally considered significant in longer-term statistical variations in signals. For a stable fiber, the PCD gives a phase bias of the delivered signal, in the way that the CD affects the phase. In LO delivery, the second-order PMD effect still needs to be studied, but is expected to have very limited effects for low PMD fiber.
For long single mode fiber, the PSPs are not necessarily linear SOPs, and the output PSPs are generally not the same as the input PSPs. Under the condition of zero Polarization Dependent Loss (PDL), the two PSPs are orthogonal to each other. In this case there is no coupling between the two PSPs—if light is launched into one of the input PSPs, then the light will not suffer polarization related temporal dispersion. However, for long/varying fiber, the PSPs are wavelength dependent, and their magnitude and orientation also vary randomly in time and wavelength, so consistently launching into an input PSP becomes difficult.
A PMD vector is also defined on the Poincare Sphere. The PMD vector has a magnitude of the DGD, and takes the direction of the PSPs. For long fiber with random perturbation, the PMD vector is a function of time, length and optical frequency. The PMD vector relates the change in output SOP S with optical frequency ω as
            ⅆ              S        _                    ⅆ      ω        =            Ω      _        ×          S      _      where S is the output SOP vector and Ω is the PMD vector.
The output SOP precesses about the PMD vector at the rate of the DGD as the frequency ω is changed. The PMD of other optical components can come from the birefringence/disturbance of any fiber pigtails, or from the component itself, such as that arising in optical isolators/circulators. Simulations have shown that the PMD in such components is due to the mismatch between different PSP paths within the component, rather than by birefringence along the same path. The PMD value of conventional optical isolators ranges from 100 fs to 500 fs. Newly developed PMD compensated optical isolators have lower PMD, typically less than 50 fs; the best on the market is specified at less than 20 fs.
Another case is for a small number of optical components cascaded together. As each component presents a section with differing birefringence, they also show partly random coupling behavior. But if the number of sections is small, and they are kept relatively stable, then no change of DGD or PSP with time should be observed. The DGD distribution characteristic is Gaussian for this kind of mode coupling.
U.S. Pat. No. 5,078,466, issued Jan. 7, 1992, is an example of a rotary joint having a pair of coaxial and radially congruent surface-defining bodies which are relatively rotatable. The bodies are flexible shape-retaining ribbon which interacts with the cable as the rotational portion of an equipped device increases or decreases the length of cable wrapped within the bodies.
U.S. Pat. No. 6,819,854, issued Nov. 16, 2004, is an example of rotary joint with a central entrance for the optical cable. A rotary hub unit contains a length of coiled cable or conduit. An angular transition cable transmits a signal between a first junction and a second end junction.
What is absent in the art is an on-axis cable wrap that allows for a prescribed range of motion of a rotating member, which smoothly communicates cable stored within the rotating member and preserves the cable signal polarization without the complexity of an electronic junction between the stationary and rotating elements.